In scanning generally flat surfaces for flaws, defects, particles and the like, a scanning beam originates from an apparent spot source, such as a reciprocating mirror or rotating polygon, i.e. an optical scanner. The scanner directs a scanning beam in an arc, but the surface prevents the scanning beam from traversing a true arc-like trajectory. If the beam had traversed a true arc, the determination of the beam position would be a simple matter. But when the beam traverses a plane the beam appears to travel faster at distances further away from the scan center. In many applications it is important to know the precise beam position. For example, in particle detection, particle position on a surface may be found only if the beam position is known. In the past, beam position could be estimated by knowing the position of the scanner and then calculating where beam should be on a surface. However, such calculations usually do not take into account factors such as wear on the scanner which cause errors relative to a theoretical scan path.
In the prior art, others have realized that scanner error creates a problem which must be corrected for precise particle or flaw position determination. For example, in U.S. Pat. No. 4,404,596 Juergensen et al. correct positional error due to uneven surfaces of a rotating polygonal mirror. While many of the prior art approaches have proved to be quite valuable, there is an ever greater need for precision, especially in locating dirt particles in ultraclea surfaces, such as semiconductor wafers. In wafer inspection, non-imaging particle detectors have been invented which accurately signal the presence of micron size particles and smaller. Mapping the location of such particles is needed to be able to predict whether circuits built on such a substrate will fail due to particle presence in a particular location.
An object of the invention is to increase the precision by which the location of a scanning beam on a surface may be determined.